Which of the following statements about the retrieval pathway from the vesicular tubular clusters to the endoplasmic reticulum (ER) are correct?
a) Proteins are transported from the Golgi apparatus to the ER via vesicles.
b) This pathway is essential for maintaining the lipid composition of the ER membrane.
c) Retrieval signals on proteins help them return from the Golgi to the ER.
d) The retrieval pathway involves the direct fusion of vesicles with the ER membrane.